1. Field of the Invention
The present invention relates to an image processing apparatus and an image processing method which form an image on a printing medium by printing a plurality of kinds of dots differing in size or density.
2. Description of the Related Art
An image printing apparatus used as output equipment such as a computer or a word processor prints an image (including characters, codes and the like) based upon image data. There are known an inkjet type, a wired dot type, a thermal type, a laser beam type, an LED type and the like as printing types.
In general, in many cases the gradation number expressible in the image printing apparatus is smaller than that of image data used in an input apparatus such as a personal computer. Therefore in many cases there is prepared the method (what is called halftone processing) for converting the gradation number of the image data used in the personal computer into the gradation number expressible in the image printing apparatus. There is known an error diffusion method or a dither method as the method of the halftone processing. The error diffusion method is advantageous in a point of being capable of acquiring an image that is high in dispersing quality, but is disadvantageous in a point of being large in processing loads. On the other hand, the dither method is a method for reducing the gradation number by comparing multi-valued image data with a threshold in advance prepared, and is smaller in processing loads than the error diffusion method. In recent years, there is proposed a method using a threshold matrix having blue noise characteristics, for realizing the dispersing quality that is equivalent to that of the error diffusion method also in the dither method.
For example, Japanese Patent Laid-Open No. 2010-260355 discloses a method in which blue noise characteristics are given to each of a plurality of dot groups that is estimated to have a physical difference, as a dot pattern to be printed in a forward pass and a dot pattern to be printed in a backward pass in a serial type printing apparatus. In addition, Japanese Patent Laid-Open No. 2013-38643 discloses a method in which in the configuration of printing in the same region of a printing medium in stages by a plurality of scans, space frequency characteristics of a dot pattern are made to blue noises in the process where dot patterns are accumulated in stages.
However, in Japanese Patent Laid-Open No. 2010-260355 or Japanese Patent Laid-Open No. 2013-38643, a case of printing a single kind of dots equal in color, size, density and the like is taken into account, but a case of printing a plurality of kinds of dots differing in color, size, density and the like is not sufficiently taken into account. As a result, there are some cases where even if the dispersing quality in the dot pattern by the single kind of dots is high, the equivalent dispersing quality cannot be obtained in the dot pattern by a plurality of kinds of dots. That is, in a case of printing a plurality of different kinds of dots, even if Japanese Patent Laid-Open No. 2010-260355 or Japanese Patent Laid-Open No. 2013-38643 is adopted, when a slight shift between dot patterns occurs, unfortunately the granularity may deteriorate to damage the image.